1. Field of The Invention
The present invention relates generally to a fixing device for an image forming apparatus, such as an electrophotographic copying machine and a laser-beam printer. More specifically, the invention relates to a fixing device for fusing and fixing a toner image to a sheet by heat and pressure.
2. Description of The Prior Art
Conventionally, image forming apparatuses, such as copying machines and laser-beam printers, are provided with a fixing device for fixing to a sheet a toner image transferred thereto. Such a fixing device comprises, for example, a fixing roller for fusing a toner on a sheet by heat, and a pressing roller for pressing the sheet against the fixing roller so as to nip the sheet therebetween. The fixing roller is cylindrical, and has a heating element which extends along the central axis of the fixing roller and which is retained therein by retaining means. The heating element is composed of, for example, a halogen lamp or the like which generates heat when a given voltage is applied thereto. Since this heating element is positioned at the central axis of the fixing roller, the heat generated by the heating element is evenly radiated to the inner wall of the fixing roller, so that the temperature distribution of the outer wall of the fixing roller is even in the circumferential direction thereof. The outer wall of the fixing roller is heated until the temperature thereof reaches a temperature suitable for fixing, for example, 150.degree. to 200.degree. C. In this condition, the fixing roller and the pressing roller rotate in the reverse directions so as to nip therebetween a sheet to which a toner has been adhered. At a contact portion where the fixing roller contacts the pressing roller, which will be hereinafter referred to as a "nip portion", the toner on the sheet is fused by the heat of the fixing roller and is fixed thereto. After fixing, the sheet is carried to a paper discharging roller by the rotations of the fixing roller and the pressing roller, and is discharged to a paper discharge tray by means of the paper discharging roller.
However, in conventional fixing devices, a lot of time is necessary for the temperature of the fixing roller to reach a temperature suitable for fixing after electric power of the apparatus, such as a laser-beam printer, has been turned on. Therefore, there is the disadvantage in that the operator can not use the apparatus for the aforementioned time, and must wait for a long time.
In this case, it is possible to decrease the time necessary to heat the fixing roller by increasing the temperature of the heating element. However, there is another disadvantage in that the demand current of the heating element increases, so that the temperature within the apparatus also increases.
Since the heating element evenly heats the inner wall of the fixing roller so that the temperature of the nip portion is the same as those of portions surrounding the nip portion, there is also the following disadvantages when the sheet is removed from the nip portion in accordance with the rotation of the fixing roller.
That is, since the toner on the sheet is not adhered and remains fused due to the heat of the fixing roller at the surrounding portion, it is difficult for the sheet to be removed from the surrounding portion other than the nip portions of the fixing roller. Consequently, there is the disadvantage in that the sheet is wound onto the fixing roller and causes jamming.